Investigation of adsorptive fractionation of humic acid on graphene oxide using fluorescence EEM-PARAFAC

Accepted Manuscript Investigation of adsorptive fractionation of humic acid on graphene oxide using fluorescence EEM-PARAFAC Bo-Mi Lee, Young-Soo Seo, Jin Hur PII:

S0043-1354(15)00040-8

DOI:

10.1016/j.watres.2015.01.020

Reference:

WR 11112

To appear in:

Water Research

Received Date: 24 September 2014 Revised Date:

9 January 2015

Accepted Date: 10 January 2015

Please cite this article as: Lee, B.-M., Seo, Y.-S., Hur, J., Investigation of adsorptive fractionation of humic acid on graphene oxide using fluorescence EEM-PARAFAC, Water Research (2015), doi: 10.1016/j.watres.2015.01.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT 1 2 3

Investigation of adsorptive fractionation of humic acid on graphene oxide

4

using fluorescence EEM-PARAFAC

Bo-Mi Leea, Young-Soo Seob, and Jin Hura,* a b

Department of Environment and Energy, Sejong University, Seoul, 143-747, South Korea Department of Nano Materials, Sejong University, Seoul, 143-747, South Korea

SC

6 7 8 9 10 11

RI PT

5

M AN U

12 13 14

Revised and Re-submitted to Water Research, January 2015

15

26 27 28 29 30 31 32

EP

* Corresponding author: Tel. +82-2-3408-3826. E-mail: [email protected]

AC C

18 19 20 21 22 23 24 25

TE D

16 17

Fax +82-2-3408-4320.

ACCEPTED MANUSCRIPT

Abstract

34

In this study, the adsorptive fractionation of a humic acid (HA, Elliott soil humic acid) on

35

graphene oxide (GO) was examined at pH 4 and 6 using absorption spectroscopy and

36

fluorescence excitation-emission matrix (EEM)-parallel factor analysis (PARAFAC). The

37

extent of the adsorption was greater at pH 4.0 than at pH 6.0. Aromatic molecules within the

38

HA were preferentially adsorbed onto the GO surface, and the preferential adsorption was

39

more pronounced at pH 6, which is above the zero point of charge of GO. A relative ratio of

40

two PARAFAC humic-like components (ex/em maxima at 270/510 nm and at (250, 265)/440

41

nm) presented an increasing trend with larger sizes of ultrafiltered humic acid fractions,

42

suggesting the potential for using fluorescence EEM-PARAFAC for tracking the changes in

43

molecular sizes of aromatic HA molecules. The individual adsorption behaviors of the two

44

humic-like components revealed that larger sized aromatic components within HA had a

45

higher adsorption affinity and more nonlinear isotherms compared to smaller sized fractions.

46

Our results demonstrated that adsorptive fractionation of HA occurred on the GO surface

47

with respect to their aromaticity and the sizes, but the degree was highly dependent on

48

solution pH as well as the amount of adsorbed HS (or available surface sites). The observed

49

adsorption behaviors were reasonably explained by a combination of different mechanisms

50

previously suggested.

SC

M AN U

TE D

EP

AC C

51

RI PT

33

52

Keywords: EEM-PARAFAC; humic substances (HS); graphene oxide (GO); adsorption; size

53

fractionation

54 55 56 57 1

ACCEPTED MANUSCRIPT 58 59

1. Introduction Graphene oxide (GO) is a representative form of graphene material that can be easily and cost-effectively obtained in substantial amounts through Hummer’s method (Kowalczuk et al.,

61

2010; Yang et al., 2013). GO is a mono-layered material consisting of partially defected

62

honeycomb structures containing a portion of various functional groups (e.g., hydroxyl,

63

epoxy, carbonyl, and carboxyl groups) (Singh et al., 2011). Owing to its unique structure, GO

64

has been used as a precursor material for a number of applications including adsorbents,

65

weak cation exchange resin, electrochemical sensors, biosensors, and hydrogen storage

66

(Ramesha et al., 2011; Singh et al., 2011; Wang et al., 2009). In the water treatment field,

67

much research has been done to utilize GO as an adsorbent for the removal of toxic

68

compounds from polluted water (Chowdhury and Balasubramanian, 2014). In addition, GO

69

can be easily modified for specific purposes either by transformation of functional groups or

70

by composition with nano-particles, or by both (Singh et al., 2011). All of these advantages

71

pose GO as one of the most attractive materials for recent environmental applications.

SC

M AN U

TE D

72

RI PT

60

Although the recent growth of GO usage heightens the probability of exposure to aquatic environments, little attention has been paid to possible environmental significance. For

74

example, GO may alter the original water quality by adsorbing certain natural or synthetic

75

organic molecules through its large surface (Wang et al., 2014a). The associated adsorption

76

mechanisms documented to date are π- π interaction, hydrophobic interaction, and

77

electrostatic attraction/repulsion (Apul et al., 2013; Sitko et al., 2013; Zhang et al., 2013;

78

Zhao et al., 2012). The sp2 structure, or honeycomb structure, of GO provides a good

79

environment for aromatic organics to adsorb through π- π interaction, while the acidic

80

functional groups on GO can be responsible for the charge-related adsorption mechanism

81

(Wang et al., 2014b). Compared with other carbon nano-materials including reduced GO and

82

carbon nanotubes (CNTs), GO can be regarded as a priority material to be examined with

AC C

EP

73

2

ACCEPTED MANUSCRIPT 83

respect to the environmental significance because it tends to be more dispersed and relatively

84

stable in the aqueous phase (Ren et al., 2014; Wang et al., 2014a).

85

Humic substance (HS) is a major constituent of naturally occurring organic materials, which are ubiquitously present in aquatic environments. HS is composed of a number of

87

aromatic and aliphatic structures of different molecular sizes and functional groups (Bell et

88

al., 2014; Hur and Schlautman, 2003). It has been reported that HS structures and their

89

composition are closely related to chemical reactivities and the subsequent environmental

90

impact (Peña-Méndez, 2005). A well-known functionality of HS is changing the surface

91

properties of adsorbents through adsorption (Kungolos et al., 2006; Liang et al., 2007; Weng

92

et al., 2006; Yang et al., 2011). The structural heterogeneity of HS can make its adsorption

93

behaviors more complicated, possibly leading to preferential adsorption of certain HS

94

constituents onto the adsorbent surfaces. There have been previous studies examining HS

95

adsorptive fractionation on natural adsorbents (e.g., minerals) and carbonaceous materials

96

(e.g., activated carbon) (Alekseeva and Zolotareva, 2013; Hur and Schlautman, 2003; Kang

97

and Xing, 2008; Schmit and Wells, 2002). Structural changes of HS upon the adsorption onto

98

CNTs were also recently studied by employing various HS characterizing techniques

99

including UV absorption spectroscopy and size exclusion chromatography (Wang et al.,

100

2011; Yang and Xing, 2009). However, to our best knowledge, there has been no related

101

report concerning GO surface, which is unfortunate considering recently growing interest in

102

GO and its potentially wide application.

SC

M AN U

TE D

EP

AC C

103

RI PT

86

For the last two decades, the fluorescence excitation-emission matrix (EEM) has been

104

established as the most popular tool for tracking the changes in the structures and the

105

composition of HS, because of its rapid measurement, non-destructive nature, and high

106

sensitivity (Li et al., 2014; Liu et al., 2011; Yamashita, 2008). Combining EEM datasets with

107

parallel factor analysis (PARAFAC) has provided even more benefits for tracking the HS 3

ACCEPTED MANUSCRIPT fluorescence features by successfully separating dissimilar fluorescent components in a more

109

quantitative manner (Cuss and Guéguen, 2012; He et al., 2013; Li et al., 2014; Simon et al.,

110

2010; Stedmon and Bro, 2008; Ziegelgruber et al., 2013). Although it looks promising for the

111

EEM-PARAFAC to be an excellent technique for exploring the changes of HS upon

112

adsorption, there have been only limited studies relying on the EEM-PARAFAC to get insight

113

into HS adsorption behaviors. The only precedent was a study of Banaitis et al., (2006), who

114

simply compared the relative distributions of four PARAFAC components of soil organic

115

matters between before and after adsorption onto minerals.

SC

RI PT

108

This study firstly examined the changes in the characteristics of HS upon the adsorption

117

onto GO. The adsorption isotherms and the kinetics were studied at two different pH values

118

(4.0 and 6.0) by using absorption spectroscopy and EEM-PARAFAC and focusing on the

119

adsorptive fractionation phenomenon. The obtained outcomes can provide a new insight into

120

the adsorption behaviors of dissimilar HS components on the GO surface and also into the

121

effects of solution pH.

122

TE D

M AN U

116

2. Materials and Methods

124

2.1. Preparation of GO

GO was prepared based via a modification of Hummer’s method (Marcano et al., 2010).

AC C

125

EP

123

126

For the preliminary step, 12 g of graphite flakes (Sigma-Aldrich) were oxidized using

127

K2SO2 (10 g), P2O5 (10 g) and 50 ml of a concentrated sulfuric acid (95%, Deajung) at 80 oC

128

for 5 hours. The pre-oxidized graphite flakes were washed with Milli-Q water and dried in

129

ambient conditions. Sodium nitrate (5 g) and dried graphite flakes (10 g) were then mixed in

130

a concentrated sulfuric acid (230 ml) at 0 oC. Potassium permanganate (30 g) was added and

131

gradually stirred into the mixture, and the solution was kept under 10 oC. The mixture was

132

heated to 35 oC and kept for 2 hours before 2 L of Milli-Q water was slowly added and 30% 4

ACCEPTED MANUSCRIPT of H2O2 (40 ml) was put into the mixture. It was stored overnight, and the supernatant was

134

decanted. The precipitated GO was washed three times with 4 N of HCl to remove

135

manganese. The GO solution was then sonicated in the acid condition for 30 minutes in a

136

bath sonicator. The GO was finally washed with Milli-Q water to remove acid and the

137

possibly remaining impurities that might exist. Single-sheet GO was finally obtained after

138

un-exfoliated GO was separated using centrifugation at 5000 rpm for 10 minutes.

RI PT

133

The GO was characterized by scanning electron microscopy (SEM, HITACHI S-4700),

140

atomic force microscopy (AFM, Park XE-100), X-ray diffraction (XRD, D/MAX-2500/PC,

141

Rigaku), Raman spectroscopy (Renisshaw 633 nm), and Fourier transform infrared

142

spectroscopy (FT-IR, Perkin-Elmer spectrum 100). The point of zero charge (pHPZC) of the

143

GO was estimated following an acid-base titration method suggested by Li et al. (2012). The

144

Brunauer−Emmer−Teller (BET) surface area of the GO is 63.6 m2/g. The GO characteristics

145

are described in the support information (Supplementary Information, Table S1; Figs. S1-S6).

146

148

2.2. HS materials

Elliott soil humic acid (ESHA) and Suwannee River fulvic acid (SRFA) were purchased

EP

147

TE D

M AN U

SC

139

from the International Humic Substance Society (IHSS). ESHA was used here as a

150

representative humic acid (HA) while SRFA, as a reference material for comparison. The data

151

of SRFA are all contained in the Supplementary Information. The aromatic, aliphatic, and

152

carboxyl contents are 50%, 16%, and 18%, respectively, for ESHA, and 24%, 33%, and 20%

153

for SRFA, respectively, based on 13C-NMR results provided by the IHSS web site

154

(www.humicsubstances.org). Using ESHA is beneficial for understanding the complicated

155

adsorption behavior of HS in aqueous phase because soil-derived HS has more heterogeneous

156

structure than aquatic HS (Hur and Schlautman, 2003).

AC C

149

157 5

ACCEPTED MANUSCRIPT 158 159

2.3. Adsorption kinetics and isotherm experiments Adsorption kinetic experiments were conducted in batch using an initial HS concentration of 15 mgC/L. The solution pH was adjusted to 4.0 and 6.0 by adding 1N NaOH. Different

161

amounts of GO were added to the pH-adjusted solutions (300 mg/L and 500 mg/L for pH 4.0

162

and 6.0, respectively). The two pH values were selected to investigate the change of

163

intermolecular interaction between HS and GO rather than to mimic the natural background

164

pH range. It is also noted that the GO concentrations used for this study are much higher than

165

the levels that might be encountered in aquatic environments. The mixtures were shaken at a

166

room temperature (20±1 oC) on an orbit shaker at 150 rpm for 5 hours in dark.

SC

M AN U

167

RI PT

160

Adsorption isotherm experiments were carried out under the same conditions. The added amounts of HS varied from 10 mgC/L to 60 mgC/L in 500 mg/L of the GO suspended

169

solution. The equilibrium time was set at 24 hours (Yang et al., 2014a). The variation of the

170

pH was minimal during shaking, varying ±0.2 from the original values. Dissolved HS was

171

separated from GO-associated fractions using centrifugation at 10,000 rpm, and the

172

supernatant was further filtered on a 0.2 µm pre-washed membrane filter (cellulose acetate

173

membrane, Advantec). The GO-associated HS or the adsorbed HS fraction was quantified by

174

subtracting the concentration of dissolved HS left in the solution from the initial

175

concentration.

177 178

EP

AC C

176

TE D

168

Adsorption isotherm parameters were estimated based on two well-known adsorption isotherm models, the Langmuir model (1) and the Freundlich model (2):

Qe =

Qmax kL Ce 1+kL Ce

(1)

1

179

Qe =kF Ce n

180

where Qe (mg C/L) and Qmax (mg C/g) are the equilibrium solid-phase concentration and the

6

(2)

ACCEPTED MANUSCRIPT maximum adsorption capacity, respectively, and Ce (mg C/L) is the equilibrium HS

182

concentration in solution. kL (L/mg C) is the adsorption affinity related to adsorption energy;

183

kF and 1/n are the Freundlich model capacity factor and the Freundlich model site

184

heterogeneity factor, an indicator of isotherm nonlinearity, respectively.

185 186

RI PT

181

The adsorption kinetic parameters were estimated by best fitting the experimental data to a pseudo first order kinetic model (3)

Qt =Qe (1-e-kt t )

(3)

SC

187

where Qt and Qe (mg C/g) are the adsorbed amounts of HS after adsorption at the time point t

189

and the equilibrium time, respectively. kt (1/h) represents the pseudo first model constant, and

190

t is adsorption time (h).

M AN U

188

191

193

2.4. Measurements of HS concentrations and the spectroscopic characteristics The initial HS and the residual HS after adsorption were quantified in dissolved organic

TE D

192

carbon (DOC) concentrations using a TOC analyzer (Shimadzu V-series, TOC-CHP).

195

Specific UV absorption (SUVA) values, a rough measure of HS aromaticity, were calculated

196

based on DOC concentration-normalized UV absorbance at 254 nm, multiplied by a factor

197

of 100. The UV absorbance was obtained using a UV-visible spectrophotometer (HACH,

198

DR 5000). Fluorescence EEMs were measured using a luminescence spectrometry (LS-55,

199

Perkin-Elmer) by emission scanning between 280 to 550 nm with 0.5 nm increment at the

200

excitation wavelengths from 250 to 500 nm at 5 nm-intervals. Excitation and emission slits

201

were adjusted at 10 nm, respectively. A 290 nm emission cut-off filter was used to limit

202

second-order Raleigh light scattering (Yang et al., 2014b). The scanning speed was set at

203

1200 nm/min. Inner-filter correction was made to account for the potential light absorption

204

of the HS samples (Gauthier et al., 1986). The obtained fluorescence intensities were

AC C

EP

194

7

ACCEPTED MANUSCRIPT normalized to units of quinine sulfate equivalents (QSE) in µg/L using the fluorescence of a

206

quinine sulfate dehydrate at an excitation/emission of 350/450 nm. All samples were

207

adjusted to the same pH condition (3.0) before the optical measurements, which makes it

208

possible to compare the adsorption behaviors at different pH without correcting the potential

209

pH-induced changes in HS (Yang and Hur, 2014). The fluorescence response to a blank

210

solution (Milli-Q water) was deducted from the measured fluorescence signals of each

211

sample.

SC

RI PT

205

212

214

2.5. EEM-PARAFAC analysis

M AN U

213

EEM-PARAFAC analysis was conducted using MATLAB 7.1 and the free-download DOMFluor toolbox (www.models.life.ku.dk), following a tutorial of Stedmon and Bro (2003).

216

The number of the PARAFAC components was determined by split-half analysis and a core

217

consistency test (Supplementary Information, Figs S7 and S8). . The EEM data for the

218

PARAFAC modeling were obtained from the HS samples (both ESHA and SRFA) before

219

and after adsorption as well as from five ultrafiltered ESHA fractions (the total number = 46).

221

EP

220

TE D

215

2.6. Ultrafiltration of ESHA and the characterization of the HA size fractions In order to obtain the structural and the spectroscopic features of HS changing with the

223

sizes, ESHA was separated into five different size fractions using an ultrafiltration process.

224

A starting HS solution was prepared by dissolving 220 mg of ESHA in l L of Milli-Q water.

225

Ultrafiltration was carried out on an Amicon stirred cell (Amicon 8400) using membranes

226

(76 mm, regenerated cellulose, Millipore, Amicon) with different molecular cut-off sizes of

227

1, 10, 30, and 100 kDa, which led to five ultrafiltered size fractions of >100 kDa, 30-100

228

kDa, 10-30 kDa, 1-10 kDa, and <1 kDa. The different molecular sizes were confirmed by

229

employing a size exclusion chromatography system (Waters 1515) equipped with a UV

AC C

222

8

ACCEPTED MANUSCRIPT detector (UV-vis 2489, Waters) and a protein-Pak 125 column (Waters). The mobile phase

231

was a pH 6.8 phosphate buffer (0.002 M NaH2PO4 and 0.002 M Na2HPO4 in 0.1 M NaCl

232

solution) (Hur and Schlautman, 2003). The ESHA fractions were further analyzed by solid

233

phase 13C-NMR measurement for the carbon structures (Bruker Avance II). Further details

234

of the ultrafiltered size fractions are provided in the supporting information (Supplementary

235

Information, Table S2; Figs. S9 and S10).

236

3. Results and Discussion

238

3.1. Adsorption isotherms of bulk HA on GO surface

239

M AN U

237

SC

RI PT

230

The adsorption isotherms of ESHA are shown in Fig. 1, and the related model parameters are presented in Table 1. The R2 values of the Langmuir and the Freundlich models were all

241

high (R2 > 0.86), suggesting that the two isotherm models reasonably explain the adsorption

242

behaviors. The Langmuir model parameters theoretically represent the maximum adsorption

243

amount (Qmax) and the related adsorption energy or adsorption affinity (kL) under certain

244

limited assumptions (i.e., monolayer adsorption, homogeneous surfaces, etc.) (Ren et al.,

245

2014), while the Freundlich model is primarily based on an empirical equation to fit the

246

adsorption data for heterogeneous surfaces (Allen et al., 2003; Zhang et al., 2013).

EP

TE D

240

For the bulk HA quantified by DOC measurements, the adsorption affinity (i.e., kL value)

248

was higher at pH 4.0 than at pH 6.0 (0.080±0.019 vs. 0.019±0.003 L/mgC). This observation

249

may be attributed to the difference in the net surface charge of GO between the pH values.

250

Due to the pHPZC of GO (~4.65) (Supplementary Information, Fig. S6), GO is likely to have

251

more positively charged (or less negatively charged) surface at pH 4.0 versus 6.0, resulting in

252

an elevated contribution of electrostatic attraction to overall adsorption and a lesser extent of

253

electrostatic repulsion between the adsorbing HA and GO surface (Moreno-Castilla, 2004).

254

Yang et al (2014a) reported a pH-dependent adsorption behavior a peat humic acid on GO,

AC C

247

9

ACCEPTED MANUSCRIPT but the extent of the adsorption with pH was not the same as that of this study (i.e., higher

256

Qmax value at a higher pH for this study) (Table 1). A closer examination of the isotherms

257

revealed that the adsorbed HA (Qe) at pH 4.0 reached a saturated level at equilibrium HA

258

concentrations over 25 mgC/L, ultimately exhibiting more nonlinear adsorption curve

259

compared to that at pH 6.0 (0.402±0.064 at pH 4.0 versus 1/n = 0.680±0.031 at pH 6.0; Table

260

1, Fig. 1a). The apparent saturation in the adsorption could result from the limitation of the

261

GO surface sites available for adsorbing HA. In contrast, relatively more linear sorption

262

isotherm at pH 6.0 may be explained by the reduced operation of the site-limiting adsorption

263

mechanisms such as electrostatic attraction. Although solution pH also influences the HA

264

properties such as the size and the molecular charges, possibly affecting the adsorption

265

behaviors, the effects seem to be limited and not strong enough to surpass the pH-induced

266

GO changes within the limited pH range of this study. It is because smaller HA size and more

267

positive charges on HA at a lower pH would result in a higher Qmax value at pH 4.0 versus 6.0,

268

which is the opposite of our results.

271

SC

M AN U

TE D

270

3.2. Adsorption isotherms of HA aromatic molecules on GO surface

EP

269

RI PT

255

To highlight the adsorption behaviors of aromatic (or UV-absorbing) moieties within HA, the isotherm model parameters were additionally estimated based on UV absorption

273

measurements (Table 1; Fig. 1b). High R2 values of the model fit indicate the relevance of the

274

isotherm models to describe the data. In this study, a higher R2 value was exhibited for the

275

UV-absorbing HS than for the bulk HS (i.e., DOC-based HS) at pH 4.0 but the same R2

276

values were found at pH 6.0 (Table 1), supporting that different mechanisms might be

277

responsible for the HA adsorption onto GO at the two pH values. Furthermore, a comparison

278

of the estimated isotherm parameters between DOC and UV absorption measurements

279

showed that the adsorption behaviors of aromatic HA were different from those of the bulk

AC C

272

10

ACCEPTED MANUSCRIPT HS. For example, the Qmax value of the UV-absorbing HA was higher and the kL value was

281

lower at pH 4.0 than at pH 6.0, while the opposite trend was previously observed for the bulk

282

HA (Table 1). In addition, the noticeably limited adsorption sites at a high adsorption amount

283

for the bulk HA were not found for this UV-absorbing HA, which exhibited a relatively linear

284

isotherm (Table 1, Fig. 1b). These results further imply that the controlling mechanisms for

285

the adsorption of aromatic HA molecules may not be the same as those of the bulk HS. The

286

adsorption of aromatic HA structures is likely to be much less affected by site-limited

287

adsorption mechanisms compared to the bulk HA. This observation appears reasonable

288

because π- π interaction, a dominant mechanism, would allow more adsorption sites for

289

aromatic HA moieties than for non-aromatic molecules, although further investigation needs

290

be warranted for better explanation.

291

293

3.3. Adsorption kinetics of HA onto GO surfaces

TE D

292

M AN U

SC

RI PT

280

Estimations of adsorption kinetic parameters, based on DOC concentrations, are presented in Table 2. The apparent equilibrium was reached within two hours (Fig. 2), which

295

agreed well with a recent report of Yang et al (2014a). The equilibrium concentration (Qe)

296

and the adsorption rate (kt) differed at the two pH conditions with the mean values being

297

approximately 2.5 and 1.6 times higher at pH 4.0 than at pH 6.0, respectively (Table 2).

298

Again, this observation can be explained by the differences in the net surface charges of GO.

299

The mean kt values obtained by UV absorption measurements were consistently higher than

300

those based on DOC (Table 2), indicating that aromatic HA moieties were adsorbed onto GO

301

surface faster than the bulk HA. The relative difference was more pronounced at a higher pH

302

(i.e., pH 6.0) where π-π interaction and hydrophobic interaction probably operate to a greater

303

extent than electrostatic attraction.

AC C

EP

294

304 11

ACCEPTED MANUSCRIPT 305 306

3.4. Changes in SUVA values upon adsorption The SUVA values of adsorbed ESHA on GO were calculated based on the mass balance between the initial and the residual ESHA after adsorption, and the changes were tracked

308

against the adsorption amounts at equilibrium (Fig. 3a) and also with the adsorption time (Fig.

309

3b). At the equilibrium condition, the SUVA values of the adsorbed HA were mostly higher

310

than the original value, suggesting that aromatic moieties within HA were preferentially

311

adsorbed to GO surface. The SUVA values showed an increasing trend with a higher

312

adsorption amount at both pH conditions (Fig. 3a). They changed from 8.5 to 10.6 L/mg C-m

313

for relatively low adsorption amounts up to ~32 mg C/g at pH 4.0, and from 11.3 to 13.8

314

L/mg C-m for all the observed adsorption amounts at pH 6.0. In addition, the differences in

315

the SUVA values between the adsorbed and the original HA were consistently larger at pH

316

6.0 than at pH 4.0 except for the high adsorption amount above ~32 mg C/g. The relatively

317

higher SUVA values indicate that the preferential adsorption of aromatic HA molecules was

318

greater at a higher pH where π- π interaction and hydrophobic interaction would be

319

predominant. For the range of the adsorbed HS amount above ~32 mg C/g at pH 4.0, the

320

SUVA values sharply deviated from a trend with the adsorbed HA amount (Fig. 3a), which

321

requires more explanation through further investigations.

EP

TE D

M AN U

SC

RI PT

307

A notable difference was also found for the kinetic changes between the two pH values

323

(Fig. 3b). At pH 6.0, the SUVA values of the adsorbed HA were initially much higher than the

324

original value and then declined with the adsorption time, varying from 12.0 to 8.7 L/mg C-m.

325

Ultimately, the values dropped nearly to the original value. In contrast, no substantial changes

326

were observed at pH 4.0 where electrostatic attraction was partially involved. From the

327

variation in the kinetics at pH 6.0 and our previous observation of the faster equilibrium for

328

the UV-absorbing HA, it can be inferred that the preferential adsorption of HA aromatic

329

moieties took place in an early stage of the adsorption, but the adsorbed aromatic molecules

AC C

322

12

ACCEPTED MANUSCRIPT 330

could be subsequently replaced with non-aromatic structures.

331

333

3.5. PARAFAC components and the relationship with HA molecular size Three different components were successfully decomposed by PARAFAC modeling on

RI PT

332

the EEM data of the samples from the adsorption and the ultrafiltration processes (Fig. 4).

335

The core consistency was 96.5% when the three-component model was applied. Several prior

336

PARAFAC studies demonstrated that three-PARAFAC component model sufficiently

337

described all the fluorescence features of HS (Borisover et al., 2012; Dainard and Guéguen,

338

2013; He et al., 2006; Yang and Hur, 2014). The peak of component 1 (C1) was shown at

339

relatively longer excitation/emission wavelengths of 270 nm/510 nm, while component 2 (C2)

340

had two maxima at shorter wavelengths of 250 nm/440 nm and 265 nm/440 nm. The two

341

PARAFAC components can be both assigned to humic-like components. Typically, the

342

fluorescence peaks at longer wavelengths (i.e., red-shifting) are associated with structural

343

condensation and polymerization of HS (Chen et al., 2003). For instance, Hur and Kim (2009)

344

have shown that fluorescence features at longer emission wavelengths were more pronounced

345

in the EEMs of larger sized HS fractions. Therefore, the C1 fluorophores probably relate to

346

more condensed structures with a larger molecular size than the C2-associated fluorescent

347

group. The peaks of component 3 (C3) appeared at <250 nm/365 nm and 290 nm/365 nm,

348

which resembled the traditionally-defined aromatic amino acid component (or tryptophan-

349

like component). In our study, the majority of fluorescent HS could be described by a

350

combination of the two humic-like components, constituting over 80% of the total

351

PARAFAC components in the Fmax values (Supplementary Information, Tables S4 and S5).

352

ESHA showed a similar relative abundance of C1 and C2 (~48%) in the Fmax values, whereas

353

C2 was more dominant for SRFA with a 70% and 20% relative abundance shown for C2 and

354

C3, respectively (Supplementary Information, Table S4). These results suggest that C2 is

AC C

EP

TE D

M AN U

SC

334

13

ACCEPTED MANUSCRIPT 355

associated with the presence of fulvic acids, which is known to be less condensed and smaller

356

in size than humic acids (Kang et al., 2002).

357

A plot of the relative ratios of the two humic-like components (i.e., C1/C2) against the weight-average molecular weights (MWw) of the ESHA ultrafiltered size fractions revealed a

359

close association of the ratios with the HA molecular sizes (Fig. 5). The C1/C2 ratios

360

generally increased with the MWw values, indicating that the ratio can be used for tracking

361

the molecular size changes of HA. The relationship was nonlinear (i.e., a concave-down

362

curve), implying that the descriptive capability of the ratio could be limited for relatively

363

large HA molecules (~ >3000 Da).

M AN U

364 365 366

SC

RI PT

358

3.6. Comparison for the adsorption isotherms between two humic-like components The adsorption isotherms of the two humic-like components (i.e., C1 and C2) were separately examined to explore the individual adsorption behaviors of different components

368

within the bulk HA (Figs. 6a and 6b). Based on the estimated isotherm parameters, the larger

369

sized humic-like component C1 exhibited the lower Qmax and 1/n values, but higher kL and kF

370

values than C2 at both pH conditions (Table 3). Our results indicate that GO has more

371

available sites for C2 than for C1. Considering that the aromatic (or fluorescent) components

372

are mostly attractive to the GO surface through π-π interaction, it is possible to infer that the

373

adsorption of larger sized aromatic molecules might be more limited on the GO surface

374

compared to smaller sized fractions. Furthermore, this tendency was more pronounced at pH

375

4.0, where electrostatic attraction could partially operate, as shown by a lower 1/n value for

376

the same components. The difference may be attributed to more operation of site-limiting

377

mechanism (i.e., electrostatic attraction) at pH 4.0. The different adsorption behaviors

378

between C1 and C2 were in a good agreement with a structural trend with the sizes of the

379

ultrafiltered fractions revealed by our 13C NMR results (Supplementary Information, Table

AC C

EP

TE D

367

14

ACCEPTED MANUSCRIPT S4), in which the larger sized fractions retain relatively high aromaticity and more abundant

381

carboxylic functional groups (i.e., more negatively charged). Comparison of the SEC

382

chromatograms before and after adsorption also supported the more site-limitation for

383

adsorption of the larger sized HA molecules as demonstrated by more adsorption of smaller

384

molecular sizes with a higher initial HA concentration (i.e., more competition)

385

(Supplementary Information, Fig S12).

386

388

3.7. Changes in C1/C2 ratios with adsorbed HS amount

Changes in the HA molecular sizes upon adsorption were tracked by the C1/C2 ratios

M AN U

387

SC

RI PT

380

against the amounts of adsorbed HA (Fig. 6c). Higher C1/C2 ratios of adsorbed HA were

390

consistently observed at pH 4.0 versus pH 6.0 for a range of the adsorption amounts,

391

indicating that preferential adsorption of C1 over C2 was more evident at pH 4.0. In detail,

392

the C1/C2 ratios were higher than the original value (1.01) at relatively low adsorbed HA

393

amounts (up to ~20 mgC/g), and they became similar to or lower than the original value as

394

the adsorbed HA amount increased. These results suggest that although preferential

395

adsorption of relatively large HS aromatic molecules occurred on the GO surface at the pH,

396

the degree became diminished as the available surface sites were occupied. At pH 6.0, the

397

C1/C2 ratios were mostly lower than the original value, implying that relatively smaller sized

398

aromatic molecules were more attractive to the GO surface. One exception was the lowest

399

adsorbed HA amount, in which the ratio was higher than the original value. Similarly to pH

400

4.0, the C1/C2 ratios showed a decreasing trend with increased adsorbed HA amount up to

401

~20 mgC/g, and they remained within a limited range above that range. Our results are in line

402

with a prior study of Yang and Xing (2009), who reported using a rough estimate of

403

molecular size based on an absorption ratio (i.e., UV250/UV365) that preferential adsorption of

404

larger molecular sized HS onto CNTs was more evident at a lower pH.

AC C

EP

TE D

389

15

ACCEPTED MANUSCRIPT 405

Taken together, our study demonstrated that the degree and the trend of HA size fractionation upon the adsorption onto GO surface can be highly dependent on the solution

407

pH and the available GO surface sites. Further investigation could provide more concrete

408

evidence for our explained mechanisms associated with the adsorptive fractionation.

RI PT

406

409 410

4. Conclusions

Adsorptive fractionation of HA on GO surface was confirmed by tracking the optical

412

properties of HA upon the adsorption. UV-absorbing HA molecules were preferentially

413

adsorbed onto GO at the two pH conditions, and they showed a faster adsorption rate

414

compared to the bulk HA. The aromatic HA moieties exhibited different adsorption behaviors

415

from the bulk HA, showing more linear isotherms and a higher Qmax and a lower kL values at

416

pH 4.0 than at pH 6.0. A positive relationship was established between the C1/C2 ratios and

417

MWw values of HA ultrafiltered size fractions, suggesting that EEM-PARAFAC could be

418

used for tracking the changes in the molecular sizes of HA. The two individual adsorption

419

isotherms of C1 and C2 revealed that the larger sized humic-like component (C1) had a

420

greater adsorption affinity but more limited adsorption sites on the GO surface. The observed

421

adsorption behaviors including the size fractionation were possibly explained by the relative

422

contributions of different mechanisms, the amount of adsorbed HA (i.e., available surface

423

sites), and a structural trend of HA with its fraction’s size.

M AN U

TE D

EP

AC C

424

SC

411

425

Acknowledgments

426

This work was supported by a National Research Foundation of Korea (NRF) grant funded

427

by the Korea government (MSIP) (No. 2014R1A2A2A09049496).

428 429

References 16

ACCEPTED MANUSCRIPT Alekseeva, T.V. and Zolotareva, B.N. (2013) Fractionation of humic acids upon adsorption on montmorillonite and palygorskite. Eurasian Soil Science 46(6), 622-634. Allen, S.J., Gan, Q., Matthews, R. and Johnson, P.A. (2003) Comparison of optimised isotherm models for basic dye adsorption by kudzu. Bioresource Technology 88(2), 143-152.

RI PT

Apul, O.G., Wang, Q.L., Zhou, Y. and Karanfil, T. (2013) Adsorption of aromatic organic contaminants by graphene nanosheets: Comparison with carbon nanotubes and activated carbon. Water Research 47(4), 1648-1654.

SC

Banaitis, M.R., Waldrip-Dail, H., Diehl, M.S., Holmes, B.C., Hunt, J.F., Lynch, R.P. and Ohno, T. (2006) Investigating sorption-driven dissolved organic matter fractionation by multidimensional fluorescence spectroscopy and PARAFAC. Journal of Colloid and Interface Science 304(1), 271-276.

M AN U

Bell, N.G.A., Murray, L., Graham, M.C. and Uhrin, D. (2014) NMR methodology for complex mixture 'separation'. Chemical Communications 50(14), 1694-1697. Borisover, M., Lordian, A. and Levy, G.J. (2012) Water-extractable soil organic matter characterization by chromophoric indicators: Effects of soil type and irrigation water quality. Geoderma 179–180, 28-37. Chen, J., LeBoeuf, E.J., Dai, S. and Gu, B. (2003) Fluorescence spectroscopic studies of natural organic matter fractions. Chemosphere 50(5), 639-647.

TE D

Chowdhury, S., Balasubramanian, R. (2014) Recent advances in the use of graphene-family nanoadsorbents for removal of toxic pollutants from wastewater. Advances in Colloid and interface Science 204, 35-56.

EP

Cuss, C.W. and Guéguen, C. (2012) Determination of relative molecular weights of fluorescent components in dissolved organic matter using asymmetrical flow field-flow fractionation and parallel factor analysis. Analytica Chimica Acta 733, 98-102. Dainard P.G., Guéguen, C. (2013) Distribution of PARAFAC modeled CDOM components in the North Pacific Ocean, Bering, Chnkchi and Beaufort Seas. Marine Chemistry 157, 216223

AC C

430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478

Gauthier, T.D., Shane, E.C., Guerin, W.F., Seitz, W.R., Grant, C.L. (1986) Fluorescence quenching method for determining equilibrium constants for polycyclic aromatic hydrocarbons binding to dissolved humic materials. Environmental Science & Technology 20, 1162–1166. He, X.-S., Xi, B.-D., Li, X., Pan, H.-W., An, D., Bai, S.-G., Li, D. and Cui, D.-Y. (2013) Fluorescence excitation–emission matrix spectra coupled with parallel factor and regional integration analysis to characterize organic matter humification. Chemosphere 93(9), 22082215. He, Z.Q., Ohno, T., Cade-Menun, B.J., Erich, M.S. and Honeycutt, C.W. (2006) Spectral and chemical characterization of phosphates associated with humic substances. Soil Science 17

ACCEPTED MANUSCRIPT Society of America Journal 70(5), 1741-1751. Hur, J. and Kim, G. (2009) Comparison of the heterogeneity within bulk sediment humic substances from a stream and reservoir via selected operational descriptors. Chemosphere 75(4), 483-490.

RI PT

Hur, J. and Schlautman, M.A. (2003) Molecular weight fractionation of humic substances by adsorption onto minerals. Journal of Colloid and Interface Science 264(2), 313-321. Kang, K.H., Shin, H.S. and Park, H. (2002) Characterization of humic substances present in landfill leachates with different landfill ages and its implications. Water Research 36(16), 4023-4032.

SC

Kang, S.H. and Xing, B.S. (2008) Humic acid fractionation upon sequential adsorption onto goethite. Langmuir 24(6), 2525-2531.

M AN U

Kowalczuk, P., Cooper, W.J., Durako, M.J., Kahn, A.E., Gonsior, M. and Young, H. (2010) Characterization of dissolved organic matter fluorescence in the South Atlantic Bight with use of PARAFAC model: Relationships between fluorescence and its components, absorption coefficients and organic carbon concentrations. Marine Chemistry 118(1–2), 22-36.

TE D

Kungolos, A., Samaras, P., Tsiridis, V., Petala, M. and Sakellaropoulos, G. (2006) Bioavailability and toxicity of heavy metals in the presence of natural organic matter. Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering 41(8), 1509-1517. Li, J., Zhang, S.W., Chen, C.L., Zhao, G.X., Yang, X., Li, J.X. and Wang, X.K. (2012) Removal of Cu(II) and Fulvic Acid by Graphene Oxide Nanosheets Decorated with Fe3O4 Nanoparticles. Acs Applied Materials & Interfaces 4(9), 4991-5000.

EP

Li, W.T., Chen, S.Y., Xu, Z.X., Li, Y., Shuang, C.D. and Li, A.M. (2014) Characterization of Dissolved Organic Matter in Municipal Wastewater Using Fluorescence PARAFAC Analysis and Chromatography Multi-Excitation/Emission Scan: A Comparative Study. Environmental Science & Technology 48(5), 2603-2609. Liang, Y.N., Britt, D.W., McLean, J.E., Sorensen, D.L. and Sims, R.C. (2007) Humic acid effect on pyrene degradation: finding an optimal range for pyrene solubility and mineralization enhancement. Applied Microbiology and Biotechnology 74(6), 1368-1375.

AC C

479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527

Liu, T., Chen, Z.L., Yu, W.Z. and You, S.J. (2011) Characterization of organic membrane foulants in a submerged membrane bioreactor with pre-ozonation using three-dimensional excitation–emission matrix fluorescence spectroscopy. Water Research 45(5), 2111-2121. Marcano, D.C., Kosynkin, D.V., Berlin, J.M., Sinitskii, A., Sun, Z.Z., Slesarev, A., Alemany, L.B., Lu, W. and Tour, J.M. (2010) Improved Synthesis of Graphene Oxide. Acs Nano 4(8), 4806-4814. Moreno-Castilla, C. (2004) Adsorption of organic molecules from aqueous solutions on carbon materials. Carbon 42(1), 83-94. 18

ACCEPTED MANUSCRIPT

Peña-Méndez, E.M., Havel, J., Patočka, J. (2005) Humic substances - compounds of still unknown structure: applications in agriculture, industry, environment, and biomedicine. Journal of applied biomedicine 3, 13-24.

RI PT

Ramesha, G.K., Kumara, A.V., Muralidhara, H.B. and Sampath, S. (2011) Graphene and graphene oxide as effective adsorbents toward anionic and cationic dyes. Journal of Colloid and Interface Science 361(1), 270-277. Ren, H., Kukarni, D.D., Kodiyath, R., Xu, W.N., Choi, I. and Tsukruk, V.V. (2014) Competitive Adsorption of Dopamine and Rhodamine 6G on the Surface of Graphene Oxide. Acs Applied Materials & Interfaces 6(4), 2459-2470.

SC

Schmit, K.H. and Wells, M.J.M. (2002) Preferential adsorption of fluorescing fulvic and humic acid components on activated carbon using flow field-flow fractionation analysis. Journal of Environmental Monitoring 4(1), 75-84.

M AN U

Simon, K.S., Pipan, T., Ohno, T. and Culver, D.C. (2010) Spatial and temporal patterns in abundance and character of dissolved organic matter in two karst aquifers. Fundamental and Applied Limnology 177(2), 12. Singh, A.P., Mishra, M., Chandra, A. and Dhawan, S.K. (2011) Graphene oxide/ferrofluid/cement composites for electromagnetic interference shielding application. Nanotechnology 22(465701), 1-9 .

TE D

Sitko, R., Zawisza, B. and Malicka, E. (2013) Graphene as a new sorbent in analytical chemistry. TrAC Trends in Analytical Chemistry 51, 33-43. Stedmon, C.A. and Bro, R. (2008) Characterizing dissolved organic matter fluorescence with parallel factor analysis: a tutorial. Limnology and Oceanography-Methods 6, 572-579.

EP

Stedmon, C.A., Markager, S. and Bro, R. (2003) Tracing dissolved organic matter in aquatic environments using a new approach to fluorescence spectroscopy. Marine Chemistry 82(3-4), 239-254. Wang, F., Haftka, J.J.H., Sinnige, T.L., Hermens, J.L.M. and Chen, W. (2014a) Adsorption of polar, nonpolar, and substituted aromatics to colloidal graphene oxide nanoparticles. Environmental Pollution 186, 226-233.

AC C

528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576

Wang, J., Chen, Z.M. and Chen, B.L. (2014b) Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets. Environmental Science & Technology 48(9), 4817-4825. Wang, L., Lee, K., Sun, Y.Y., Lucking, M., Chen, Z.F., Zhao, J.J. and Zhang, S.B.B. (2009) Graphene Oxide as an Ideal Substrate for Hydrogen Storage. Acs Nano 3(10), 2995-3000. Wang, X.L., Shu, L., Wang, Y.Q., Xu, B.B., Bai, Y.C., Tao, S. and Xing, B.S. (2011) Sorption of Peat Humic Acids to Multi-Walled Carbon Nanotubes. Environmental Science & Technology 45(21), 9276-9283. 19

ACCEPTED MANUSCRIPT

Weng, L.P., Van Riemsdijk, W.H., Koopal, L.K. and Hiemstra, T. (2006) Adsorption of humic substances on goethite: Comparison between humic acids and fulvic acids. Environmental Science & Technology 40(24), 7494-7500.

RI PT

Yamashita, Y. (2008) Assessing the dynamics of dissolved organic matter (DOM) in coastal environments by excitation emission matrix fluorescence and parallel factor analysis (EEMPARAFAC). Limnology and Oceanography 53(5), 1900-1908. Yang, K. and Xing, B.S. (2009) Adsorption of fulvic acid by carbon nanotubes from water. Environmental Pollution 157(4), 1095-1100.

SC

Yang, L. and Hur, J. (2014) Critical evaluation of spectroscopic indices for organic matter source tracing via end member mixing analysis based on two contrasting sources. Water Research 59, 80-89.

M AN U

Yang, L.Y., Shin, H.S. and Hur, J. (2014b) Estimating the Concentration and Biodegradability of Organic Matter in 22 Wastewater Treatment Plants Using Fluorescence Excitation Emission Matrices and Parallel Factor Analysis. Sensors 14(1), 1771-1786. Yang, S., Li, L., Pei, Z., Li, C., Shan, X.-q., Wen, B., Zhang, S., Zheng, L., Zhang, J., Xie, Y. and Huang, R. (2014a) Effects of humic acid on copper adsorption onto few-layer reduced graphene oxide and few-layer graphene oxide. Carbon 75, 227-235.

TE D

Yang, S.B., Hu, J., Chen, C.L., Shao, D.D. and Wang, X.K. (2011) Mutual Effects of Pb(II) and Humic Acid Adsorption on Multiwalled Carbon Nanotubes/Polyacrylamide Composites from Aqueous Solutions. Environmental Science & Technology 45(8), 3621-3627.

EP

Yang, Z., Yan, H., Yang, H., Li, H., Li, A. and Cheng, R. (2013) Flocculation performance and mechanism of graphene oxide for removal of various contaminants from water. Water Research 47(9), 3037-3046. Zhang, C.L., Wu, L., Cai, D.Q., Zhang, C.Y., Wang, N., Zhang, J. and Wu, Z.Y. (2013) Adsorption of Polycyclic Aromatic Hydrocarbons (Fluoranthene and Anthracenemethanol) by Functional Graphene Oxide and Removal by pH and Temperature-Sensitive Coagulation. Acs Applied Materials & Interfaces 5(11), 4783-4790.

AC C

577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621

Zhao, G.X., Wen, T., Yang, X., Yang, S.B., Liao, J.L., Hu, J., Shao, D.D. and Wang, X.K. (2012) Preconcentration of U(VI) ions on few-layered graphene oxide nanosheets from aqueous solutions. Dalton Transactions 41(20), 6182-6188. Ziegelgruber, K.L., Zeng, T., Arnold, W.A. and Chin, Y.P. (2013) Sources and composition of sediment pore-water dissolved organic matter in prairie pothole lakes. Limnology and Oceanography 58(3), 1136-1146.

20

ACCEPTED MANUSCRIPT

Tables Table 1. Isotherm model parameters of ESHA adsorption. Langmuir pH Qmax kL R2

UV

kF

1/nd

R2

4

47.9±4.1a

0.080±0.019b

0.91

8.43±1.77c

0.402±0.064

0.86

6

69.0±7.0a

0.019±0.003b

0.99

2.41±0.26c

0.680±0.031

0.99

4

710±290e

0.122±0.063f

0.96

77.56±4.09g

0.832±0.067

0.97

RI PT

DOC

Freundlich

M AN U

SC

6 417±42e 0.167±0.023f 0.99 60.30±1.55g 0.771±0.029 0.99 a the maximum adsorption capacity estimated based on DOC measurement (mg C/g) b the adsorption affinity estimated based on DOC measurement (L/mg C) c the Freundlich model capacity estimated based on DOC measurement ((mg C/g)(L/mg)1/n) d the Freundlich intensity factor, an indicator of isotherm nonlinearity (dimensionless) e the maximum adsorption capacity estimated based on UV absorption measurement (1/cm·g) f the adsorption affinity estimated based on UV absorption measurement (cm) g the Freundlich model capacity estimated based on UV absorption measurement ((1/cm·g)(cm)1/n) h Not measured

DOC

a

AC C

UV

pH

Qe

kt

R2

4.0

16.67±0.27a

8.77±1.00b

0.96

6.0

7.73±0.26a

5.54±0.97b

0.87

4.0

0.821±0.016c

9.69±1.46b

1.00

6.0

c

EP

Measurements

TE D

Table 2. Parameters for the pseudo first-order model.

0.618±0.007

22.03±9.73

b

Adsorbed amount in DOC concentrations at equilibrium condition (mg C/g) Pseudo first model constant (1/h) c Adsorbed amount in UV absorbance (at 254 nm) at equilibrium condition (1/cm·g) b

1.00

ACCEPTED MANUSCRIPT Table 3. Isotherm model parameters for each PARAFAC component.

4.0

6.0

Langmuir

comp-

Freundlich kF

1/ng

R2

0.97

7.34±0.52c

0.57±0.02

0.99

0.025±0.005b

0.98

5.22±0.52c

0.70±0.03

0.99

135±23a

0.017±0.004b

0.98

3.57±0.48c

0.74±0.04

0.98

a

b

0.99

c

0.81±0.04

0.99

onents

Qmax

kL

R

C1

87±8a

0.046±0.008b

C2

128±18a

C1 C2

187±29

0.013±0.003

a

2

RI PT

pH

3.36±0.40

the maximum adsorption capacity of PARAFAC components (QSE/g) the adsorption affinity of PARAFAC components (L/QSE) c the Freundlich constant of PARAFAC components ((QSE/g)(L/mg)1/n)

AC C

EP

TE D

M AN U

SC

b

1

ACCEPTED MANUSCRIPT

45

(a)

ESHA pH 4.0

40

ESHA pH 6.0 Langmuir

35

Freundlich

25

RI PT

Q e (mg C/g)

30

20 15

5 0 10

20

30

40

M AN U

0

SC

10

50

Ce (mg C/L)

250 ESHA pH 4.0 ESHA pH 6.0 Langmuir Freundlich

TE D

150

100

AC C

50

EP

Qe (1/cm g)

200

(b)

0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Ce (1/cm)

Fig. 1. Adsorption isotherms of ESHA on GO at 0.1 M NaCl based on (a) DOC measurements and (b) UV absorption measurements (UV254).

1

ACCEPTED MANUSCRIPT

25 ESHA pH 4.0

(a)

ESHA pH 6.0

20

15

RI PT

Qt (mg C/g)

Pseudo first order

10

0 1

2

3

4

5

M AN U

0

SC

5

6

Adsorption time (hr)

1.0

TE D

0.6

0.4

ESHA pH 4.0

EP

Q t (1/cm g)

0.8

0.2

AC C

(b)

ESHA pH 6.0 Pseudo first order

0.0

0

1

2

3

4

5

6

Adsorption time (hr)

Fig. 2. Adsorption kinetics of ESHA on GO at 0.01 M NaCl based on (a) DOC measurements and (b) UV absorption measurements. Error bars represent triplicate samples. The composite samples were used for the absorption and the fluorescence measurements.

1

20

(a)

RI PT

18 16 14 12 10

SC

8 6

ESHA pH 4.0

4

ESHA pH 6.0

2

M AN U

SUVA values of adsorbed ESHA (L/mg C·m)

ACCEPTED MANUSCRIPT

ESHA

0 0

10

20

30

40

13

(b)

ESHA pH 4.0

TE D

12 11

ESHA pH 6.0 ESHA

EP

10 9 8

AC C

SUVA values of adsorbed ESHA (L/mg C·m)

Adsorbed amount of ESHA (mg C/g)

7 6

0

1

2

3

4

5

6

Adsorption time (hr)

Fig. 3. Changes in the SUVA values of adsorbed ESHA with (a) adsorption amount at equilibrium condition and (b) adsorption times (adsorption kinetics).

1

ACCEPTED MANUSCRIPT 500

500

(a)

(b)

(C)

350

0 2e-2 3e-2 4e-2 5e-2 6e-2 7e-2

300

400

350 0 2e-2 3e-2 4e-2 5e-2 6e-2 7e-2

300

250 350

400

450

500

550

Emission (nm)

350

300

250 300

400

250 300

350

400

450

Emission (nm)

500

550

0 4.0e-2 6.0e-2 8.0e-2 1.0e-1 1.2e-1

RI PT

400

450

Excitation (nm)

450

Excitation (nm)

450

300

350

400

SC M AN U TE D EP 1

450

Emission (nm)

Fig. 4. PARAFAC components (a) C1, (b) C2, and (c) C3.

AC C

Excitation (nm)

500

500

550

ACCEPTED MANUSCRIPT

3.0 2.5

RI PT

C1/C2 ratios

2.0 1.5

SC

1.0

0.0 0

2000

M AN U

0.5

4000 MWW (Da)

6000

8000

Fig. 5. A relationship between the ratios of the two PARAFAC components (C1/C2) and

AC C

EP

TE D

weight-averaged molecular weights for five ultrafiltered size fractions of ESHA.

1

ACCEPTED MANUSCRIPT

70

(a) 60

40

RI PT

Qe (QSE/g)

50

30

C1

20

C2

SC

Langmuir

10

Freundlich

0 10

20

30

40

M AN U

0

Ce (QSE)

70

(b) 60

TE D

40 30

C1

EP

Qe (QSE/g)

50

20

C2 Langmuir

AC C

10

Freundlich

0

0

10

20 Ce (QSE)

1

30

40

ACCEPTED MANUSCRIPT

(c) 1.1

RI PT

1.0 0.9 0.8

ESHA pH 4.0

0.7

ESHA pH 6.0

0.6

SC

C1/C2 ratios of adsorbed ESHA

1.2

ESHA

10

20

30

M AN U

0

40

Adsorbed amount of ESHA (mg C/g)

Fig. 6. Adsorption isotherms of C1 and C2 at (a) pH 4.0 and (b) pH 6.0, and (c) changes of

AC C

EP

TE D

C1/C2 ratios of adsorbed ESHA with the adsorption amounts on GO.

2

ACCEPTED MANUSCRIPT

Highlights

► EEM-PARAFAC was successfully applied to examine HS adsorptive behavior on

RI PT

graphene oxide (GO).

SC

► Aromatic molecules within HS were preferentially adsorbed onto GO surface.

higher isotherm nonlinearity.

M AN U

► Larger sized fluorescent component exhibited a greater adsorption affinity and a

AC C

EP

TE D

► Adsorptive fractionation depends on solution pH and available sites on GO.

ACCEPTED MANUSCRIPT

Supplementary Information Water Research

1 2 3 4 5

Investigation of adsorptive fractionation of humic acid on graphene oxide

7

using fluorescence EEM-PARAFAC

RI PT

6

8

b

Department of Environment and Energy, Sejong University, Seoul, 143-747, South Korea Department of Nano Materials Engineering, Sejong University, Seoul, 143-747, South Korea

M AN U

a

SC

Bo-Mi Leea, Young-Soo Seob, and Jin Hura,*

EP

TE D

* Corresponding author: Tel. +82-2-3408-3826. E-mail: [email protected]

AC C

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

1

Fax +82-2-3408-4320.

ACCEPTED MANUSCRIPT 47 48

S1. Characterization of GO To confirm the synthesis of GO, the structural features of graphite and the synthesized GO were compared each other by employing a variety of the characterizing tools including

50

scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction

51

(XRD), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). It was

52

clearly seen that the graphite retained stacked graphene layers, while the exfoliated GO was

53

wrinkled in its thin layer (Fig. S1a). From the AFM scanning (Fig. S2), it was confirmed that

54

our synthesized GO consists of the single layers with its thichness of approximately 1 nm.

55

Similar GO structure with thin layers was also reported in other previous studies (Cote et al.,

56

2009; Sun et al., 2012). The XRD spectrum of GO showed a prominant peak at lower degree

57

at 10.82°. In contrast, there were two sharp peaks at 26.34° and 54.52° for graphite (Fig. S3).

58

The peak at the lower degree is associated with longer distances between the atoms within the

59

structures, suggesting that the the adjacent layers of the GO are placed in longer distances

60

between one another than those of the graphite. For example, the degree of the GO peak

61

corresponds to 0.822 nm while those of the graphite reflect 0.340 nm and 0.169 nm,

62

respectively. Moon et al. also reported the similar results of 0.330 nm and 0.860 nm for

63

graphite and GO, respectively (Moon et al., 2010).

SC

M AN U

TE D

EP

In the Raman spectra (Fig. S4), graphene peak (G peak at 1580 cm-1) and two-dimension

AC C

64

RI PT

49

65

peak (2D peak at 2700 cm-1) were observed for graphite, while the GO showed a defected

66

peak (D peak at 1336 cm-1), the G peak, and an decresed 2D peak. The formation of D peak

67

indicates that the sp2 structure may be partially defected by strong oxidation (Moon et al.,

68

2010). However, the defection does not appear to be unsual because our defected rate (0.86)

69

base on the relative intensities of the D peak to the G peak (ID/IG) fell within the range

70

previously reported (Moon et al., 2010; Wang et al., 2014). According to Acik et al. (2010), 2

ACCEPTED MANUSCRIPT 71

the FT-IR peaks of GO are typically shown at the wavenumber ranges associated with the

72

vibration of epoxide (C-O-C) (1230-1320 cm-1), sp2-hydridized C=C (1500-1600 cm-1),

73

carboxyl (COOH) (1650-1750 cm-1), ketonic species (C=O) (1600-1650 cm-1, 1750-1850 cm-

74

1

75

similar peaks at 1134 cm-1, 1261 cm-1, 1408 cm-1, 1640 cm-1, 1731 cm-1, 3218 cm-1, and 3401

76

cm-1 for the GO , indicating that the GO can be characterized by sp2 structure and the

77

functional groups of epoxy, carboxy, and hydroxyl. The point of zero charge of the GO in this

78

study was estimated to be 4.65 based on an acid-base titration (Fig. S6) (Zhao et al., 2012).

SC

RI PT

), and hydroxyl (C-OH) (3050-3800 cm-1 and 1070 cm-1). Our FT-IR result showed the

81

AC C

82

Fig. S1. Images of scanning electron microscopy (SEM) for (a) graphite and (b) GO.

EP

80

TE D

M AN U

79

83 84

Fig. S2. AFM image of GO. 3

ACCEPTED MANUSCRIPT

10.82˚

RI PT

Arb. intensity

26.34˚

SC

GO

54.52˚

10

20

30

40

50

M AN U

0

Graphite 60

70

2 θ (°)

85 86

Fig. S3. Comparison of the XRD images for graphite and GO in this study.

TE D

87

2D-band

AC C

Arb. intensity

EP

D-band G-band

500

1000

GO

Graphite 1500

2000

2500

3000

3500

Raman Shift (cm -1) 88 89

Fig. S4. Comparison of the Raman spectra for graphite and GO in this study. 4

ACCEPTED MANUSCRIPT

C-O-C

COOH

-OH

C=C

RI PT

Arb. intensity

C=O

SC

GO

Graphite

3500

3000

2500

2000

1500

M AN U

4000

1000

500

Wavenumber (cm -1)

90 91

Fig. S5. Comparison of the FT-IR spectra for graphite and GO in this study.

93 94

TE D

92

Table S1. Calculated atomic distance of graphite and GO based on the XRD spectra. Graphite

95 96

54.52

10.82

0.34

0.17

0.82

AC C

d (nm)

26.34

EP

2θ ( ˚ )

GO

5

ACCEPTED MANUSCRIPT

0.0015

0.0010

pHpzc=4.65

RI PT

TOTH

0.0005

0.0000

-0.0010

-0.0015 4

6

M AN U

2

SC

-0.0005

8

pH

97 98

Fig. S6. Acid-base titration curve for GO.

100 101

120 100.0

96.5

EP

80

100.0

AC C

Core consistency (%)

100

TE D

99

60 40

17.7

20

0.1 0 1 102 103

2

3 Component No.

4

Fig. S7. Result of core consistency test (n=46). 6

5

ACCEPTED MANUSCRIPT 104 105 106

0.3

(a)

0.25

RI PT

ex split1 ex split2 ex split3 ex split4 em split1 em split2 em split3 em split4

Loading

0.2 0.15

SC

0.1

0 250

300

M AN U

0.05

350

400

450

500

550

Wavelenth (nm)

107

0.4

TE D

0.35 0.3

0.2 0.15

AC C

0.1

ex split1 ex split2 ex split3 ex split4 em split1 em split2 em split3 em split4

EP

Loading

0.25

(b)

0.05 0

250

300

350

400 Wavelenth (nm)

108

7

450

500

550

ACCEPTED MANUSCRIPT

0.45

(c)

0.4 0.35

ex split1 ex split2 ex split3 ex split4 em split1 em split2 em split3 em split4

0.25

RI PT

Loading

0.3

0.2 0.15

0.05 0 300

350

400

450

M AN U

250

SC

0.1

500

550

Wavelenth (nm)

109 110 111 112

Fig. S8. The split half test results as PARAFAC excitation and emission loadings of (a) C1, (b) C2 and (c) C3.

113

Table S3. Adsorption model parameters for SRFA models

Isotherm

EP

Isotherm

AC C

Freundlich

Kinetic 114 115 116 117 118 119 120 121 122

DOC measurements

UV254 measurements

Qmax

36.0±6.9a

-e

kL

0.010±0.002b

-e

R2

0.98

-e

kF

0.54±0.06c

-e

1/n

0.791±0.033d

-e

R2

0.99

-e

Qe

9.65±0.09f

0.383±0.011h

kt

5.77±0.36g

8.98±3.00g

Parameters

TE D

Adsorption experiments

Pseudo-first model

R2 0.97 0.99 a the maximum adsorption capacity estimated based on DOC measurement (mg C/g) b the adsorption affinity estimated based on DOC measurement (L/mg C) c the Freundlich model capacity estimated based on DOC measurement ((mg C/g)(L/mg)1/n) d the Freundlich intensity factor, an indicator of isotherm nonlinearity (dimensionless) e Not measured f Adsorbed amount in DOC concentrations at equilibrium condition (mg C/g) g Pseudo first model constant (1/h) h Adsorbed amount in UV absorbance (at 254 nm) at equilibrium condition (1/cm·g) 8

ACCEPTED MANUSCRIPT 123

S2. Ultrafiltration of ESHA and the characterization of the HS size fractions The chatacteristics of the five ultrafiltered size fractions of ESHA are presented in Table

125

S2. The size fractions of > 30 kDa consititues nearly 88% of the total DOC pool of ESHA

126

(Table S2). In general, the larger ultrafiltered fractions exhibited higher SUVA, higher

127

aromaticity (aromatic carbon/aliphatic carbon), higher C1/C2 ratios, and more abaundance of

128

carboxyl functional groups than smaller sized fractions (Table S2). A portion of the size

129

exclusion (SEC) chromatograms of the different size fractions were overlapped with each

130

other, but the size fractionation through the ultrafiltration processes could be confirmed by

131

the shorter retention times of the highest peaks for the obtained size fractions in the order of

132

under 1 kDa (10.27 min) > 1-10 kDa (9.73 min) > 10-30 kDa (9.33 min) > 30-100 kDa (8.92

133

min) > over 100 kDa (6.03 min) (Fig. S7). In this study, the highest peak at humic-like

134

fluroescence region of the excitation-emission spectra tends to be shifted toward longer

135

wavelengths for larger ESHA sized fractions (Fig. S8). This osbservation agreed well with a

136

previous report based on humic substances extracted from soils and sediments (Hur and Kim,

137

2009). It was previously demonstrated that the red-shifting is typically associated with more

138

condensed and polymerized structures of humic substances (Chen et al., 2003; Sierra et al.,

139

2005).

141 142 143

SC

M AN U

TE D

EP

AC C

140

RI PT

124

144 145 146

9

ACCEPTED MANUSCRIPT Table S4. Characteristics of different ultrafiltered size fractions of ESHA Distri-

SUVA

bution

(L/mg C-

(%)

m)

1>

2.93

3.69

1-10

1.29

10-30

Size

13

MWw

C-NMR C1/C2

Carboxyl

Aliphatic C

Aromatic C

(%)

(%)

(%)

1105

9.4

40.2

47.3

1.18

0.23

7.00

1349

9.1

42.1

46.8

1.11

0.76

7.25

8.74

1883

11.4

33.8

51.7

1.53

1.56

30-100

30.5

9.51

3639

14.0

30.0

51.8

1.73

2.11

100 <

58.1

8.63

7165

13.2

30.4

52.5

1.73

2.46

(kDa)

(Da)

Aromaticity

RI PT

147

Note that the dissimilarity in the molecular weight values defined by the two fractionation methods (i.e., smaller MW for SEC vs. ultrafiltration) is attributed to the differences in the two systems including the molecular weight standard, separation principles, environmental conditions (Li et al., 2004).

156

residual HS for adsorption kinetics.

M AN U

SC

148 149 150 151 152 153 154 155

Table S4. Changes in the relative abundance of C1, C2, and C3 in the Fmax values (%) of

ESHA

4.0

AC C

6.0

SRFA

4.0

%C1 48.0 43.9 42.9 42.6 42.9 42.3

Fmax constitution (%) %C2 47.5 44.7 46.8 46.9 46.9 47.8

%C3 5.0 13.9 11.7 11.5 11.3 10.8

0.0 0.2 0.5 1.0 2.0 5.0

48.0 33.4 34.8 34.4 33.9 34.1

47.5 59.0 59.7 57.6 58.3 58.3

5.0 6.7 4.8 7.2 7.0 6.7

0.0

6.6

68.8

24.6

0.3 0.6

5.9 5.2

67.6 73.4

26.5 21.4

1.1 2.1 5.0

9.7 5.8 5.7

68.9 72.7 72.8

21.4 21.6 21.5

adsorption time (hr) 0.0 0.2 0.5 1.0 2.0 5.0

TE D

pH

EP

HS

157 10

ACCEPTED MANUSCRIPT 158

Table S5. Changes in the relative abundance of C1, C2, and C3 in the Fmax values (%) of

159

residual ESHA for the adsorption isotherms.

EP

161 162

AC C

160

TE D

6.0

%C2 47.5 44.5 48.3 47.9 46.9 47.1 49.3 46.8 46.6 46.3 45.5 46.2 42.0 43.1 41.9 42.8 39.3 38.5 42.9 43.1 43.3 38.5 43.7

11

%C3 5.0 19.2 7.3 5.9 5.3 6.4 6.4 5.8 5.0 5.5 6.0 4.6 18.4 13.8 14.8 12.0 17.3 19.8 10.9 10.7 8.9 19.8 9.9

RI PT

100 10 15 20 25 30 35 40 45 50 55 60 10 15 20 25 30 35 40 45 50 55 60

%C1 48.0 36.3 44.5 46.2 47.8 46.5 44.2 47.5 48.4 48.2 48.5 49.1 39.6 43.2 43.4 45.2 43.4 41.8 46.2 46.2 47.9 41.7 46.4

SC

original 4.0

Fmax constitution (%)

Initial conc. (mg C/L)

M AN U

pH

ACCEPTED MANUSCRIPT

> 1 kDa

0.0012

1-10 kDa 10-30 kDa

0.0010

30-100 kDa

RI PT

< 100 kDa

0.0008 0.0006

SC

0.0004 0.0002 0.0000 10

100

M AN U

DOC-normalized UV signal (at 254 nm)

0.0014

1000

10000

100000

Molecular Weight (Da)

163

EP

TE D

Fig. S9. SEC chromatograms of different ultrafiltered size fractions (UV-D at 254 nm)

AC C

164

12

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

165 166

Fig. S10. Fluorescence excitation-emission matrices of different ultrafiltered size fractions of

167

(a) <1 kDa, (b) 1-10 kDa, (c) 10-30 kDa, (d) 30-100 kDa, and (e) >100 kDa. 13

ACCEPTED MANUSCRIPT 168

14

(a)

DOC

12

Langmuir Freundlich

RI PT

Q e (mg C/g)

10 8 6

SC

4 2

0

M AN U

0 10

20

30

40

50

Ce (mg C/L)

14 12

(b)

0.400

TE D

0.350

8 6

0.300 0.250 0.200 0.150

EP

Qt (mg C/g)

10

0.450

4

AC C

2

DOC

0.100

UV at 254 nm

0.050

Pseudo first order

0

0

1

Qt (1/cm·g)

169

0.000 2

3

4

5

6

Adsorption time (hr)

170 171

Fig. S11. (a) Adsorption isotherms of SRFA by DOC measurements and (b) adsorption

172

kinetic of SRFA by DOC measurements and UV absorption measurements (UV254) at pH 4.0.

173

(The amount of GO used is 500 mg/L).

174 175 14

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

176

177 178

Fig S12. Molecular weight distributions of adsorbed ESHA in percentages. (a) pH 4.0, (b) pH

179

6.0. The samples were obtained from adsorption isotherm experiments. The numbers in the

180

parentheses indicate the percent removals in DOC by adsorption. Please note that site-limited

181

adsorption (or competition) is more pronounced with a higher concentration of the initial HA

182

(or at a lower removal percentage).

183 15

ACCEPTED MANUSCRIPT 184

References

185 186 187

Acik, M., Lee, G., Mattevi, C., Chhowalla, M., Cho, K. and Chabal, Y.J. (2010) Unusual infrared-absorption mechanism in thermally reduced graphene oxide. Nature Materials 9(10), 840-845.

188

Chen, J., LeBoeuf, E.J., Dai, S. and Gu, B. (2003) Fluorescence spectroscopic studies of natural organic matter fractions. Chemosphere 50(5), 639-647.

RI PT

189 190 191 192 193

Cote, L.J., Kim, F. and Huang, J.X. (2009) Langmuir-Blodgett Assembly of Graphite Oxide Single Layers. Journal of the American Chemical Society 131(3), 1043-1049.

194

SC

Hur, J. and Kim, G. (2009) Comparison of the heterogeneity within bulk sediment humic substances from a stream and reservoir via selected operational descriptors. Chemosphere 75(4), 483-490.

M AN U

195 196 197 198 199 200 201 202 203

Li, L., Zhao, Z., Huang, W., Peng, P., Sheng, G. and Fu, J. (2004) Characterization of humic acids fractionated by ultrafiltration. Organic Geochemistry 35, 1025-1037. Moon, I.K., Lee, J., Ruoff, R.S. and Lee, H. (2010) Reduced graphene oxide by chemical graphitization. Nature Communications 1(73).

204

212 213 214 215 216 217 218 219

TE D

209 210 211

Sun, W.L., Xia, J., Li, S. and Sun, F. (2012) Effect of natural organic matter (NOM) on Cu(II) adsorption by multi-walled carbon nanotubes: Relationship with NOM properties. Chemical Engineering Journal 200–202, 627-636.

EP

208

Sierra, M.M.D., Giovanela, M., Parlanti, E. and Soriano-Sierra, E.J. (2005) Fluorescence fingerprint of fulvic and humic acids from varied origins as viewed by single-scan and excitation/emission matrix techniques. Chemosphere 58(6), 715-733.

Wang, F., Haftka, J.J.H., Sinnige, T.L., Hermens, J.L.M. and Chen, W. (2014) Adsorption of polar, nonpolar, and substituted aromatics to colloidal graphene oxide nanoparticles. Environmental Pollution 186(0), 226-233.

AC C

205 206 207

Zhao, G.X., Wen, T., Yang, X., Yang, S.B., Liao, J.L., Hu, J., Shao, D.D. and Wang, X.K. (2012) Preconcentration of U(VI) ions on few-layered graphene oxide nanosheets from aqueous solutions. Dalton Transactions 41(20), 6182-6188.

220 221 222 223

16